Establishment of a connection on the GSTN between two facsimile devices, and the transfer of data from one to the other according to T.4 and T.30 is at present performed by modems working in an unusual half-duplex mode; that is, high-speed facsimile data is transmitted in one direction alternately with low-speed control and acknowledgement signals in the other direction. As a result, the high-speed receiver must be retrained before each batch of high-speed data can be sent; as the speed capabilities of modems are continually improved the time required for training becomes an increasingly significant proportion of the total transmission time.
Furthermore, there will be periods of time when no signals in either direction are present in the network, and the connection may be re-routed; this would further increase the time needed for retraining.